The values exported from exif.js are put on a global object named EXIF (incidentally the same as your import). In a normal browser environment you would access them by using EXIF.Tags, EXIF.getTag etc.
But in QML javascript globals are imported into their own namespace (in your case also EXIF).
So anything declared on the global context inside the exif.js is available on that additional EXIF object.

So - to make complicated things short:

To access your functions you have to call @EXIF.EXIF.getTag@.

You could also change exif.js and remove the closure and have all the functions and constants directly on the global context.